A conventional X-ray CT device has been known, which irradiates a subject to be examined with X-rays, detects X-rays transmitted through the subject or scattered by the subject with an X-ray detector, and provides a fluoroscopic image, tomogram, or three-dimensional image of the subject on the basis of the X-ray detection output (X-ray photon count).
A cone beam CT device has recently been developed as such an X-ray CT device. A general X-ray CT device uses an X-ray beam formed thin in the Z direction (this beam is a fan beam). In contrast to this, a cone beam CT device uses an X-ray beam which also spreads in the Z direction (this X-ray beam is called a cone beam).
Studies have recently been undertaken to realize a conventional CT device (i.e., having only one row) as this cone beam CT device in the form equivalent to a so-called third-generation type or a scheme called an R/R type. The third-generation type CT is designed to perform scanning (acquisition of projection data) while making a pair of an X-ray source and a detector pivot around a subject to be examined.
FIG. 2 is a view showing an example of a cone beam CT device. The cone beam CT device shown in FIG. 2 belongs to third-generation type CT devices, and makes both an X-ray source and an X-ray detector pivot about the Z-axis around a subject to be examined. They make one rotation to complete scanning of a region of interest. In a general X-ray CT device, detection elements are arranged in one line in a channel (CH) direction for sampling. Each element is identified by a channel number. In contrast to this, in a cone beam CT device, as shown in FIG. 2, detection elements are also arranged in the Z direction (row direction). That is, the detector of the cone beam CT device has detection elements two-dimensionally arranged in the form of an orthogonal lattice. According to such a cone beam CT device, detection elements are arranged in the two directions, i.e., the Z direction (row direction) and CH direction, in the form of a lattice to form a detector, and radiation is applied in the form of a cone by making it have a thickness in the Z direction as well, thereby obtaining projection data corresponding to a plurality of columns at once.
With regard to the above cone beam CT devices, a device using a flat panel detector (to be referred to as an FPD hereinafter) has been expected to be commercialized. Unlike a conventional one-dimensional sensor, an FPD which is a two-dimensional sensor allows interference of scattered radiation. This may cause a decrease in contrast.
In a cone beam CT device, in order to remove scattered radiation, division plates made of lead or tungsten are inserted between the detection elements in the CH direction to separate them from each other, and blades (slits) are inserted between the detection elements to separate them from each other in the Z-axis direction. In X-ray imaging using an FPD, a grid for scattered radiation removal is generally used as disclosed in Japanese Patent Laid-Open No. 2001-212139. The blades are inserted between the detectors to separate them from each other in the Z direction. That is, the detector pitch in the Z direction is equal to the blade pitch. On the other hand, the grid is placed in front of the detectors, and hence may shield the detectors. This causes a shadow on an image.
Japanese Patent Laid-Open No. 2000-107162 discloses a technique of inserting a plurality of blades, which are inserted in the Z-axis direction to remove scattered radiation, between the respective detection elements for separation in the Z-axis direction. This reference also discloses a technique of moving a plurality of blade modules arranged in the Z direction over the detectors of a fourth-generation CT (a CT device having detectors arranged in the form of a ring in the CH direction).
Japanese Patent Laid-Open No. 2000-157530 discloses a technique in which a grid similar to the one used for X-ray imaging to remove scattered radiation is used in a cone beam CT device. According to Japanese Patent Laid-Open No. 2000-157530, in order to prevent moire in acquired data which is caused when the grid is inserted, the grid is moved in a direction perpendicular to the extending direction of grid elements to prevent any grid image from remaining during X-ray imaging.
Japanese Patent Laid-Open No. 2001-188096 discloses a structure in which lattice-like grooves are formed in the glass substrate of an FPD, and the grooves are filled with an X-ray absorbing material, thereby reducing scattered radiation.
Japanese Patent Laid-Open No. 2001-212139 discloses a technique of performing data processing to reduce grid patterns from an image obtained by X-ray imaging using a fixed grid.
As described above, when a cone beam CT device is to be formed, blades are inserted in the Z-axis direction, as disclosed in Japanese Patent Laid-Open No. 2000-107162. In a cone beam CT device using an FPD, however, it is mechanically difficult to insert blades between the respective detectors in the Z-axis direction, as described in Japanese Patent Laid-Open No. 2000-107162. Such a structure is expensive, even if possible. This is because, when an FPD is used, detection elements corresponding to several hundred channels (CHs) may need to be arranged in the Z-axis direction as well.
As proposed in Japanese Patent Laid-Open No. 2000-157530, therefore, a grid for scattered radiation may be applied to a CT device. However, the grid directly blocks part of X-rays while cutting scattered X-rays. The grid blocks X-rays in accordance with the arrangement of the grid. This generally causes damage to an image in the form of a grid pattern. In CT, since inverse projection is performed in the form of rotation, if a grid pattern remains in an X-ray image, a ring-like false image (artifact) occurs upon image reconstruction. According to Japanese Patent Laid-Open No. 2000-157530, therefore, a grid moving mechanism is provided to move the grid during X-ray imaging to remove a grid shadow from an X-ray image. However, a very special mechanism is required to move the grid in the form of an arc. When the grid is to be moved in the Z-axis direction, the grid must be moved in a direction perpendicular to the direction of rotational motion, resulting in resonance and the like. This technique is therefore impractical.
Although Japanese Patent Laid-Open No. 2001-188096 discloses a two-dimensional radiation detector provided with a lattice-like X-ray absorbing member to remove scattered X-rays, there is no description about an arrangement using an FPD and grid. Although Japanese Patent Laid-Open No. 2001-212139 discloses an X-ray imaging device for simple imaging using an FPD and grid, there is no description about a CT device.